Representatives of all vertebrate classes produce antibodies which resemble those of man in polypeptide structure and diversity. Although protostomate invertebrates apparently lack the capacity to form antibodies, insufficient information is currently available on the primitive chordates which are ancestral to vertebrates to delineate precisely their immunological status. Investigations of immunity in protochordates are warranted because evidence exists that certain species can recognize tissue grafts from members of the same species. These protochordate species may represent a living example of the primordial state of the veterbrate immune system which will allow detailed elucidation of the mechanisms of immune recognition, generation of diversity in recognition specificity, and antigen directed differentiation towards cellular or humoral immunity. This research will be directed towards investigating the cellular recognition and immunological capacities of tunicates, cephalochordates, and hemichordates. We will use techniques involving the binding of radioactively labelled antigens to determine the presence of blood cells which show binding capacity for foreign antigenic molecules. We will apply techniques which we have developed for the enzymatic iodination of surface proteins of lymphocytes and procedures of protein chemistry to isolate these immunoglobulin-like surface receptors from the surfaces of lymphocyte-like cells of these species. The isolated surface receptor molecules will be compared to immunoglobulins of lower vertebrate species and mammals by serological and biochemical techniques. The results should prove relevant to the origins of antibodies as well as to the emergence of cell-mediated immune reactions including rejection of allografts and tumors.